what corrosion inhibitor?

[malakin]

I am designing marine structures in the caribbean. I understand that it is advisable to use a corrosion inhibitor. DCI has been suggested. DCI is a calcium nitrite (anodic inhibitor) based corrosion inhibitor. I also understand that laboratory studies have found the rate of corrosion of steel to be accelerated when calcium chloride and other chemicals are used as accelerators. What corrosion inhibitor should I use?

Kind regards,
M

 

[Ron]

DCI should work well. Keep in mind that your aggregates likely have a high chloride content so set your dosage to accommodate.

 

[Hariharan]

Malakin,

1.Never use any item containing chlorides in marine concrete.
Chlorides are the main chemicals causing corrosion in rebars.
2. Why do you need accelerators? Normal setting concrete
should be OK!
3. We use blast furnace slag cement. Reduces pores and
chloride ingress. Some types of Pozzolona (Sp?) cements are
also acceptable. These are slow setting cements.
4. Several design and construction requirements should be
followed to get durable marine concrete. I haven't found out
a single reference or code giving all the requirements in one
place yet! Please research out before you get into design or
construction.

Hariharan

 

[Gourile]

Agreed with Harriharan. Also, you should use 50kg/m3 more cement for a concrete that is poured in sea water. This increases durability and your setting time so that usually you won't need any accelerator. Avoid using cement type V as it is weak against cloride ione, try using cement typeII instead of that. Epoxy coated reinforcement is a good solution, but your local code should be checked to see if this kind is permitted. In case of using this type of reinforcement, the development lengthes should be reanalysed.

 

[Hariharan]

You should define 'Marine environment'. Is it for components
above water, but in a salt laden atmosphere, or is it for
components which are subject to alternate wetting and drying,
like tidal/splash zone, or for components permanently
submerged in water?

I would be skeptical about using epoxy coated rebars in any
marine environment. There have been a number of publications
regarding the long term performance of ECR bars, and recent
evidence is not in favour of ECR bars in marine environment,
specifically if there is water ingress. Water can permeate thru
the epoxy, with the result that the bond between steel
and epoxy is lost over time, and that is very serious. Further,
strict quality control is required to avoid damages to coating
at various stages. It could be more of a pain than a boon!

The basic requirements for concrete for marine environment
are:

1. Higher grade: say 40 Mpa
2. Maximum water/ cementitious material ratio < 0.4
3. Minimum cementitious material > 350 kg/m^3
4. Use of blast furnace slag or fly ash or other pozzolana -
Pozzolana should be tested to be suitable for the environment
This addition results in a less permeable concrete at a
reduced consumption of cement.
5. Clear cover to reinforcement 50mm for beams, and
75 mm for piles, drilled shafts, portions in water/soil
6. Limit on maximum stress in reinforcement under &quot;normal&quot;
conditions, i.e. excluding extreme environmental
conditions. If the code specifies any limits on crack width,
it should be met.
7. In some areas protective coatings on concrete surface are
provided. This is inspectable and maintainable, unlike
coatings on rebars.

Anything above this should be evaluated for cost- benefits.

Discussion on the above would be welcome!

Hariharan